Chip-Scale Atomic Clocks (CSACs) include vapor cells that contain vapors of an alkali metal such as rubidium (Rb). The vapor cells also typically contain a buffer gas, such as an argon-nitrogen buffer gas blend. The standard technique for fabricating the vapor cells involves anodically bonding two glass wafers on opposing sides of a silicon wafer having a plurality of cell structures that define cavities. The alkali metal vapor and buffer gas are trapped in the cavities of the cell structures between the two glass wafers.
The anodic bond joint starts at the locations between the wafers that are initially in contact and spreads out as the electrostatic potential brings the surfaces together. This lag of the bond front from one area to the next can lead to pressure differences in the vapor cells. Additionally, the presence of a low boiling temperature material like Rb requires the bonding to take place at as low a temperature as possible, otherwise the vapor generated can foul the bond surface. Thus, a high voltage needs to be applied as the wafers are heating, to allow the bond to form as soon as possible. This can result in vapor cells sealing at different times, and thus at different temperatures, which can result in pressure differences in the vapor cells, even on cells that are fabricated side-by-side on the same wafer.
Further, total thickness variations in the two glass wafers cause some of the vapor cells to become hermetically sealed before other vapor cells on the same set of wafers. This problem is further exacerbated in that the temperature is gradually ramped in the bonder equipment, driving some of the trapped gas out of vapor cells that bond late. In addition, there are no easy escape paths for buffer gas that gets trapped in regions that bond late, which can lead to pressure differences in the vapor cells.
Lastly, due to the presence of the buffer gas, the voltage that is applied to accomplish anodic bonding can create a breakdown of the gas, causing a discharge or arc through the gas to ground, essentially shorting out the bonding process.